1. Field of the Invention
The present invention relates to a method of treating compression ignition engine exhaust gases such as diesel engine exhaust gases. More particularly, the present invention relates to a method which utilizes a catalyst to convert NO in diesel exhaust gas streams to NO2 at low temperatures. The NO2 (nitrogen dioxide) generated by the catalyst can then be used to promote regeneration of a downstream diesel particulate filter at low temperatures.
2. Background of the Invention
In recent years, environmental regulations in the United States and Europe for diesel particulate emissions have required the effective removal of particulate (generally consisting of carbonaceous particulates in the form of soot) from the diesel engine. A well-known method for removing soot from engine exhaust has been to collect the soot using a particulate filter or porous ceramic trap made from cordierite or SiC followed by oxidation of accumulated particulate at the high temperatures encountered in the exhaust.
However, lean-burn engines such as diesel engines operate at much cooler temperatures than gasoline engines. As a result, the lower temperature of the exhaust gases hinders the oxidation of particulate captured in the particulate filter. The use of catalytic traps capable of promoting particulate filtration and regeneration have been proposed as one solution to this problem. It has also been found that the presence of NO2 in diesel exhaust results in oxidation of particulate at lower temperatures up to 300xc2x0 C. NO2 reaction with soot occurs even at room temperature, but the reaction time takes up to 2 orders of magnitude longer. Accordingly, it would be desirable to be able to use a catalyst to convert NO in diesel exhaust gas to NO2 at the lowest possible temperature in order to increase the rate of particulate combustion.
Diesel catalysts have been proposed containing platinum, which is active at temperatures below about 300xc2x0 C. Among catalysts studied, the Pt/silica catalyst exhibited the best activity; however, it exhibited low activity below 250xc2x0 C. The same catalyst was the most active in particulate combustion due to the effective NO oxidation to NO2. There are thermodynamic limitations of NO conversion to NO2 at temperatures exceeding 250xc2x0 C. While it is impossible to achieve high conversion of NO to NO2 at higher temperatures due to the thermodynamic limitations, the development of catalysts which are active at temperatures between about 175 to 250xc2x0 C. could lead to practically complete conversion. It would be desirable to improve the activity of catalysts at low temperatures and improve the thermal stability of catalysts for NO oxidation to NO2.
Accordingly, there is still a need in the art for a method of treating compression ignition exhaust gases such as diesel engine exhaust gases which can successfully convert NO to NO2 at temperatures of less than 250xc2x0 C.
The present invention meets that need by providing an improved method for converting NO in diesel engine exhaust gases to NO2 at low temperatures utilizing an oxidation catalyst comprising platinum and a support material comprising zirconia-silica. The resulting NO2 promotes regeneration of the diesel particulate filter (i.e., combustion of trapped carbonaceous particulate) at a temperature less than about 250xc2x0 C., or accumulates on NOx trap material for particulate combustion at higher temperatures.
According to one aspect of the present invention, a diesel exhaust gas treatment system is provided comprising an oxidation catalyst positioned in an exhaust gas passage of a diesel engine for converting at least a portion of NO contained in the exhaust gas to NO2. The oxidation catalyst comprises platinum and a support material comprising zirconia-silica. The system also includes a particulate filter for receiving the exhaust gas containing NO2, where the NO2 enables oxidation of particulate trapped on the filter, thereby promoting regeneration of the filter.
The conversion of NO to NO2 preferably occurs at a temperature range of between about 175 to 350xc2x0 C., and more preferably at a temperature of between about 200 and 250xc2x0 C. The oxidation of particulate preferably occurs at a temperature less than about 250xc2x0 C.
The oxidation catalyst is preferably positioned between the exhaust passage and the particulate filter. The oxidation catalyst preferably comprises from about 1 to 5 wt. % platinum on the support. The support contains from about 3 to 20 wt. % zirconia, and the balance silica. To increase the activity of the catalyst, or decrease Pt loading at the same level of performance, the catalyst also preferably includes one or more oxides selected from the group consisting of TiO2, P2O5, WO3, B2O3, and Al2O3, or a heteropolyacid selected from H3PW12O40 and H4SiW12O40.
The system may optionally include a second catalyst positioned downstream from the particulate filter for NOx reduction. The second catalyst preferably comprises a selective reduction catalyst, which is capable of reducing nitrogen oxides such as NO2 and NO into nitrogen (N2) or nitrous oxide (N2O). In addition, reductants such as ammonia, urea, or diesel fuel may be added to the second catalyst to aid in reduction.
In another embodiment of the invention, the system may include a NOx trap positioned downstream from the oxidation catalyst. This NOx trap may be combined with a particulate filter according to the method developed by Toyota Motor Corp. (Automotive Engineering International/October 2000, p.119) for simultaneous removal of NOx and particulate. According to another aspect of the present invention, a method for treating diesel exhaust gases is provided in which an oxidation catalyst is positioned in the exhaust gas passage of a diesel engine, where the catalyst comprises platinum and a support material comprising zirconia-silica. The oxidation catalyst is exposed to diesel exhaust gas such that at least a portion of NO contained in the exhaust gas is converted to NO2. The NO2 is then passed through a particulate filter provided downstream of the oxidation catalyst in an amount sufficient to oxidize particulate trapped on the filter.
The method preferably includes pretreating the oxidation catalyst in a gas mixture containing NO, O2 and N2 prior to positioning the catalyst in the exhaust stream. The gas mixture preferably comprises about 500 ppm of NO, about 3% by volume O2, and the balance N2. The pretreatment is preferably carried out a temperature of between about 500 to 650xc2x0 C.
The method also preferably includes adding one or more oxides (such as, for example, TiO2, P2O5, WO3, B2O3, and Al2O3) to the catalyst to further improve the performance prior to positioning the catalyst in the exhaust stream, or adding a heteropolyacid such as, for example, H3PW12O40 and H4SiW12O40.
The method of the present invention results in a high conversion of NO contained in the exhaust gas to NO2 of from about 60% to about 96% at a temperature of about 175 to 250xc2x0 C.
The method may further include providing a second catalyst downstream of the particulate filter to aid in reducing NOx. The second catalyst is preferably a selective reduction catalyst as discussed above.
The method may also include positioning an NOx trap downstream from the oxidation catalyst (between the oxidation catalyst and the particulate filter) or combining the trap with the particulate filter.
Accordingly, it is a feature of the present invention to provide a diesel exhaust treatment system and method for converting NO to NO2 at low temperatures. This, and other features and advantages of the present invention, will become apparent from the following detailed description, the accompanying drawings, and the appended claims.